Gas filter devices protect their user from the inhalation of gases and vapors that are hazardous to health. This protection lasts until the uptake capacity of the adsorption filter is exhausted. The time period from the start of use of the filter insert to the point in time at which relevant concentrations of the components to be separated by the filter appear on the inhalation side is called the breakthrough time or retention time of the adsorption filter. The filter retention time depends on a number of parameters, including, e.g., the type and the amount of the adsorbent contained in the filter, the filter design, the type of the toxic substance, the concentration of the toxic substance, environmental conditions, such as the temperature and the relative humidity, and the respiratory minute volume of the user of the device. To effectively protect the user of the device, the user must be warned by a suitable measuring and warning means in time before the breakthrough of the filter. If this is not possible, it is absolutely necessary to have an idea of the retention time of the adsorption filter used under the conditions of use. To ensure this, either the environmental parameters and the filter parameters must be known, or, if they are not known, they must be estimated for the worst case.
Various procedures have been known for indicating a necessary filter change. Substances that can be noticed from their odor in time can be recognized by the user at the time of breakthrough. Consequently, this odor perception can be used to indicate a necessary filter change. The drawback of the use of such odor perception is that the faculty of perception may be subject to great variations both from one person to the next and depending on the personal health status. Filter breakthrough recognition that is based on odor perception alone is therefore no longer permissible in some countries. In addition, the user of the device should have a certain time available after the indication to leave the hazardous area and to carry out the filter change. This possibility is not given in cases in which warning is generated only after the filter breakthrough.
Even though the drawback of the individual variations and of the variations based on the personal well-being is eliminated if the subjective odor perception is replaced with a sensor system located in the breathing connection, which recognizes and activates a warning, the drawback of the lack of a safety margin in time until the beginning of the breakthrough is not eliminated. This drawback can be eliminated by analyzing the air flowing through the filter bed at a point within the filter bed, e.g., at 80% to 90% of the height of the filter bed. Such a device has been known from WO 96/12524. The drawback of this is that interventions in the filter design are necessary and that the filters become larger and heavier due to the sensor system and the electronic system to be installed.
If none of the above-mentioned possibilities is used to recognize a filter breakthrough, the protection of the user must be guaranteed by developing use guidelines which are specific of the particular workplace and in which the service lives are regulated.
The filter service lives specified by the use guidelines are based, on the one hand, on the determination of the conditions of use and the environmental conditions and, on the other hand, either on measured values, which were obtained with the filters under the conditions of use determined, or on filter retention times theoretically derived for the environmental conditions determined. The drawback of the use of such use guidelines is that the indication of a necessary filter change is not based on the currently occurring conditions, but the worst case was used for the determination of the service life. As a result, the filters are used very insufficiently in most cases.